Technical Field
The present invention relates to a gas sensor element that is mounted on a vehicle, for example, and detects the concentration of oxygen in exhaust gas.
Background Art
In a variety of industries, a variety of attempts has been made worldwide to reduce environmental impacts and burdens. In particular, in the automobile industry, development for promoting the spread of not only fuel-efficient gasoline engine vehicles, but also so-called eco-friendly vehicles, such as hybrid vehicles or electric vehicles, as well as for further improving the performance of such vehicles has been advanced day by day.
Purification of exhaust gas and improvement of fuel economy performance of vehicles have been conducted by detecting the concentration of oxygen in a measurement target gas, such as exhaust gas, using a gas sensor and precisely controlling the fuel oil consumption and the intake air amount.
An exemplary basic configuration of a gas sensor element that constitutes such a gas sensor includes a detection portion, which has a stack of a solid electrolyte body having a pair of electrodes on opposite sides thereof and a heat generating body including a heat generating source, and a porous protective layer formed around the detection portion.
A gas sensor detects the concentration of oxygen in exhaust gas at a temperature as high as about 400 to 850° C. Thus, if water droplets (i.e., condensed water) in the exhaust gas collide with the gas sensor element that constitute the gas sensor, it is concerned that thermal shock may be generated due to partial quenching, and due to a change in the volume of the element with a change in the temperature, an abnormal output resulting from a temperature drop when the element becomes wet may be generated or an abnormal output resulting from cracking of the element due to the thermal shock may be generated. Further, it is also concerned that metallic compounds in the condensed water may infiltrate the element together with the water, which in turn can poison the detection portion of the gas sensor element.
In order to eliminate such concerns, a porous protective layer is provided around the detection portion of the gas sensor element.
Herein, as the conventional art related to a gas sensor element with a porous protective layer, Patent Document 1 discloses a gas sensor element in which the periphery of the element is surrounded by a porous protective layer made of alumina to suppress collision of water droplets. In addition, Patent Document 2 discloses a gas sensor element with a porous protective layer that is made of a single material of silicon carbide or aluminum nitride or a mixed material thereof with other ceramic materials. Further, Patent Document 3 discloses a gas sensor element that has, on a gas introduction outer surface, through which a measurement target gas is introduced, of a diffusive resistance layer of the main body portion of the element, a porous protective layer for trapping poisonous components in the measurement target gas, and a surface protective layer that is formed on the porous protective layer, has water repellency at a high temperature at which a solid electrolyte body becomes active, and has smaller porosity than the porous protective layer.
As described above, a variety of improvements has been made to a porous protective layer that is provided around a detection portion to improve the water-resistant property of gas sensor elements. In particular, a technology related to a material that is applied to the porous protective layer has been mainly developed. It should be noted that in Patent Document 3, the surface roughness of the surface protective layer, which has smaller porosity than the porous protective layer, is defined to prevent cracking of the element, which would otherwise occur due to thermal shock when the element becomes wet, whereby the water repellency of the surface protective layer can be ensured.
By configuring a porous protective layer of a gas sensor element such that it repels condensed water in exhaust gas, it becomes possible to significantly reduce thermal shock that can be generated in the porous protective layer or the gas sensor element and also suppress infiltration of the condensed water, thereby solving the problem that the detection portion may become poisoned.
Regarding the water repellency of the porous protective layer, the inventors have found that a porous protective layer with excellent water repellency can be obtained through a different approach from that of the conventional art.